1. Field of the Invention
The present invention relates to a Y-type hexagonal oxide magnetic material and an inductor element composed of this material. More particularly, the present invention relates to a Y-type hexagonal oxide magnetic material and an inductor element which can be advantageously applied for use in a high frequency range.
2. Description of the Related Art
In a high frequency circuit for use in various mobile communication devices such as a portable telephone and a wireless LAN, an inductor element with an air core coil structure, such as a chip inductor, is used to cover frequency ranges up to several GHz for various purposes including impedance matching, resonating and choking.
An air core coil is formed by winding a wire around a core made of a nonmagnetic material or by forming a coil pattern on a substrate made of a nonmagnetic material. Accordingly, it has a disadvantage in that it is necessary to increase the number of coil turns when a larger inductance is required. This is an impediment to the miniaturization of the inductor element. Furthermore, since the resistance of the wire grows in proportion to the number of coil turns, there is another problem in that it is not possible to provide an inductor having a high Q (gain) factor.
The Q factor represents a value obtained by dividing the real number part (xcexcxe2x80x2) of the magnetic permeability by the imaginary number part (xcexcxe2x80x3).
On the other hand, a hexagonal ferrite is known as a magnetic material for high frequency use which has an easy axis of magnetization in the surface perpendicular to the c axis. The magnetic permeability of the hexagonal ferrite having this in-plane magnetic anisotropy is attenuated at a frequency higher than that of the frequency limit (Snoek peak) of a spinel ferrite represented by a Nixe2x80x94Znxe2x80x94Cu ferrite. Accordingly, it is possible to reduce the number of coil turns in comparison with an air core coil when this ferrite is used, providing a possibility for miniaturizing an inductor element having such a coil.
However, even when a Ferrox planar type ferrite (which belongs to a group of hexagonal ferrites known to have the best high frequency characteristics) sintered compact is used, a frequency relaxation phenomenon caused by magnetic domain wall motion is observed, with the result that the frequency at which a high Q factor can be maintained is limited to the level of about 200 MHZ. Accordingly, a large improvement in the characteristics cannot be achieved in comparison with a spinel ferrite.
Accordingly, it is one of the objects of the present invention to provide a Y-type hexagonal oxide magnetic material which has a large magnetic permeability in the frequency range of from several MHZ to several GHz while maintaining a high Q factor.
Another object of the present invention is to provide a small-sized inductor element with a high Q factor using the above-described Y-type hexagonal oxide magnetic material.
To solve the above-described technical problems, the Y-type hexagonal oxide magnetic material according to the present invention is characterized in that it comprises at least Fe, Co and M (where M is at least one of Ba and Sr) as well as O, wherein the relationship of x+3"sgr"xe2x89xa64, preferably x+3"sgr" less than 4, is satisfied, in which x represents the average grain size (xcexcm) of a sintered compact thereof and a represents the standard deviation of said grain size.
It is preferable that the Y-type hexagonal oxide magnetic material according to the present invention is obtained from a starting raw material by the steps comprising blending, calcination, grinding, molding and baking, where for the raw material, a hydroxide or a carbonate is used for the M, an iron oxide is used for the Fe, and an oxide or a carbonate is used for the Co, and that the ground powder obtained after the grinding step has a specific surface area of not less than about 5 m2/g.
Furthermore, the Y-type hexagonal oxide magnetic material according to the present invention is preferably obtained from a Y-type hexagonal oxide magnetic powder calcined by a spray roasting process.
In the Y-type hexagonal oxide magnetic material according to the present invention, part of the Co may be replaced with at least one of Cu, Zn and Ni.
The present invention is also directed to an inductor element having a magnetic body composed of such a Y-type hexagonal oxide magnetic material as described above.
Other objects, advantages, and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawing and examples.